Sure, a modern SBC repackaged in an all-in-one case can be a good option for most of the first group I mentioned. However, out of the box it doesn't boot to BASIC like an early 80s home computer so you'd need to configure a custom boot and choose a language. And even within that group there are some for who the simplicity of limited graphics, resolution and other capabilities is an important element, so they'd probably want to go further to something like auto-booting directly into a software emulation of an 80s computer.

For those who care about running retro software titles, software or FPGA emulation would be the minimum, however that doesn't help those who want support for retro physical media or peripherals, CRT displays or highly accurate emulation. You may not care about those things but for a significant part of the addressable market for machines like this, one or more of those things are a high priority. Which was my point, the retro-interested market looks pretty large but when you zoom in, you realize it's fragmented and that makes it challenging to design a device that appeals to enough buyers. Since the RPi 400 seems perfect to you, you probably don't care about it looking like a C64, however the folks behind this new device paid a lot to buy the Commodore brand name and logo (supposedly up to a million dollars). And that's before the plastic moulds, retro keyboard etc. For you, that may all be wasted money and effort. For others it might be a big part of the appeal.

It's certainly possible to make a retro computer that has HDMI, Wifi, Ethernet and SD cards and gets on the modern web but also has analog composite, S-video, and component output for CRTs, along with having a cartridge port, floppy drive, serial or parallel port and which runs period software with cycle-accuracy. However, that device is probably going to cost close to $1,000 and take a fair bit of time and expertise to create. I'm a hardcore retro-enthusiast and I'd certainly consider spending that kind of money but most people probably wouldn't. Worse... I'm not even in the market for such a wonder-machine because I already own over a hundred different models of retro computer along with analog composite, S-video and composite CRTs to use them (almost every 80s model of Commodore, Atari, Apple, Amiga, Sinclair, Amstrad and Radio Shack along with other more obscure brands).